"The cure for boredom is curiosity. There is no cure for curiosity." -Dorothy Parker

polymer

Big science news out of the University of Calgary: researchers have found that BPS impacts brain behaviour in zebrafish! You may have seen it in the new this week: it made it on CBC, the Toronto Star, the National Post and many other news sites. Some contained alarming headlines like “Attention pregnant shoppers: study says those cash register receipts could harm your unborn child. (National Post)” But really, is that what the study said? The title, published in Proceedings of the National Academy of Sciences, reads “Low-dose exposure to bisphenol A and replacement bisphenol S induces precocious hypothalamic neurogenesis in embryonic zebrafish” Seems a bit of a reach to say that you may be harming your unborn child, doesn’t it? And here lies some of the challenges in science communication: making the study accessible to the public, without oversimplifying or over reaching the conclusions. It is a tough balance. This study helps highlight those challenges and it also gives an opportunity to explain how these studies are really important to communicate; how they impact our understanding of our human health and development; and the impact that we have on the world around us.

This study, led by Dr. Deborah Kurrasch at the University of Calgary, is pretty interesting science. What’s more, is that her and her team said they were surprised by the results. So let’s have a look at what Dr. Kurrasch’s team discovered.

Figure 1: chemical structure of bisphenol A

The story begins with the compound bisphenol A, abbreviated as BPA. It wasn’t long ago that BPA was making the headlines: it was dangerous and has since been banned from baby bottles. Many of us are now purchasing BPA-free water bottles, baby bottles, plastic packing etc. So what is BPA anyway? It is the compound here on the right. It is used in making plastics, most commonly polycarbonate plastics, but also used in epoxy resins. These are just various types of plastics that have all kinds of uses. Soon it became apparent that we were finding BPA EVERYWHERE. In all kinds of places it shouldn’t be. As someone who studied polymers, I can say what surprised me about this compound is that, unlike some other additives to plastics, it is chemically bonded right into the polymer. The fact that it was leaching out was a bit of a surprise. But what is worse is that BPA behaves as an endocrine disruptor. Endocrine disruptors are a group of compounds that behave similarly to hormones in your body. In this case, BPA behaves similarly to thyroid hormones.

The endocrine system is one of the systems in your body that is responsible for communication and regulation of body functions such as growth, reproduction, metabolism, and behaviour. Hormones are the chemicals produced by your endocrine cells and secreted and transported throughout the body as the chemical communicators. Chemicals that are structurally similar to hormones can bond to hormone receptors in your body and therefore disrupt the communications, which can cause problems with your body’s development.

Figure 2: chemical structure of bisphenol S

Now knowing that BPA is an endrocrine disruptor, it became apparent that it should be removed from materials. The trick is to find something else that can function in place of BPA, but won’t pose the same problem. Manufacturers started using bisphenol S, or BPS, in place of BPA. (BPS is shown here on the right.) This compound is used in place of BPA, meaning that BPA-free products may contain BPS in its place.

So we know that both of these compounds are present in the environment, we know that there are found in organisms, that they bioaccumulate, and that they are endrocrine disruptors. Great, so how does this actually impact you? Well…we don’t exactly know. Enter: Dr. Kurrasch and her team.

The study uses zebrafish (Figure 3) as a model organism (Model organisms are other living things that are used in experiments because it would be unethical to test on human subjects.) Zebrafish actually have a spinal cord and a brain, but also reproduce rapidly, making them good models for neural development. (The rapid reproduction allows you to get enough subjects so your results actually mean something without waiting a hundred years.)

Figure 3: zebrafish

Using VERY low doses of bisphenol a and bisphenol s (1000 times less than typical human daily exposure), the team treated the zebrafish embryos and found that they were producing more neurons (brain cells) during development, which resulted in hyperactivity in the zebrafish babies.

The studies on human development are still emerging, meaning that we don’t yet know the long term impacts on human health, or the exact mechanism of how these chemicals work. These studies from the University of Calgary help give in sight into those mechanism and also suggest that it isn’t just BPA that is a problem, but chemicals that are the same class. It also suggests that very minute amounts of these compounds can have an impact on brain development.

Take home messages of this study:

-BPA and its relative BPS both impact the brain development of zebrafish embryos

-Only minute quantities are required to see an observable difference in brain development of zebrafish

-It may be important consider removing all bisphenol compounds from consumer materials

-The impact on human health is still unknown

-These results do give in sight on potential mechanisms on human brain development

Well winter has arrived here in Alberta! Right now it seems as though in Calgary we can expect a nice high pressure system to move in and make winter balmy. But this is just a small reprieve. One of the biggest challenges of the Alberta winter is driving around. Ice an snow make roads like something out of Mario Kart. So how can chemistry help you survive winter? With the science of winter tires! Why are winter tires mandatory in Quebec? Why are some Albertans lobbying for the same law in this province? Are winter tires that important? Well, anyone I have asked have all stated that they love winter tires and are shocked at the difference it has made. The difference all comes down to glass transition temperature (Tg).

Take a look around your home. I am sure that you can find numerous examples of different types of plastics. Some are rubbery, some are hard, some are fiberous. These characteristics are going to determine how different polymers (plastics are a type of polymer) are going to be used. Now think of a plastic bucket. The kind that you may have used as a kid to build sandcastles. That thing was indestructible during the summer, but leave it outside in Calgary on a day when the high is -28 °C and drop it, that same bucket would shatter into a million pieces. What we are observing is a change in “state” of the polymer. Now this might sound odd, considering it is still solid, and the states of matter are solid, liquid, and gas. So how can we be seeing a change in state? Enter the glass transition.

Polymers can have two solid states: they can be glassy; these are hard plastics, like cellphone cases and water bottles; or they can be rubbery; these are flexible plastics, like rubber balls, or tires. The glass transition temperature (Tg) is the temperature at which a polymer switches between the glassy state and the rubbery state. If a polymer is used BELOW its glass transition temperature, it will be glassy or hard. If a polymer is used ABOVE its glass transition temperature, it will be rubbery or flexible. The polycarbonate water bottle on my desk is an example of a plastic that I am using BELOW its glass transition temperature, while the flexible silicone spatula I used to make my breakfast is an example of a plastic I am using ABOVE its glass transition temperature. Going back to the plastic bucket example: In the winter time, the bucket is below its Tg, making it glassy, and more fragile, so it breaks.

At cold temperatures, rubber tires are also going to go through this change. Rubber tires were such a great advancement (thank you John Boyd Dunlop) in the tire because these air-filled rubber tires absorbed shock, had more contact area with the road surface, and consequently, gave more traction. The more a tire interacts with the road, the more traction a vehicle has. In the snowy, icy winter, we need all the traction we can get. To get a nice, flexible tire that has lots of contact with the road, it needs to be used above its Tg. However, in Canada, our winters are going to push that. Our -40 °C winter days are going to bring a regular tire down to, if not crossing, its Tg. This will make it more rigid, and therefore, it will have less contact with the road surface, which will decrease the traction, precisely at a time when drivers want MORE traction. Also, the treads on the tire will become less flexible, allowing for snow to build up in them, further reducing traction.

Winter tires are made of a type of rubber that has a much lower Tg than summer tires or all season tires. This means that even as the mercury drops, the tire will not approach the Tg, and will stay flexible, resulting in more road contact, less snow build up, and MORE TRACTION. More traction means less sliding, smaller stopping distances, and safer driving. Enjoy safer winter driving thanks to the chemistry of polymers and the glass transition temperature. Get yourself some winter tires.